Poly(vinyl alcohol) (hereinafter PVA) fiber has been spotlighted as textile and industrial fibers for more than sixty years since it was first prepared by Herrmann in 1931 [German Patent No. 685,048].
There are two types of PVA used as a raw material of PVA fiber having high tensile strength and high tensile modulus. The first type is a tactic PVA, with the following formula (a), and the other is syndiotactic PVA, represented by the following formula (b).
Since the syndiotactic PVA has structural stability due to zigzag-type molecular structure, the syndiotactic PVA fiber has better mechanical property, thermal stability, chemical and weather resistance, when compared to the atactic PVA fiber. ##STR1##
In general, in the conventional preparative method of PVA fiber, the spinning, drawing, and heat treatment processes are included to give high orientation to the molecular chain. But the syndiotactic PVA has difficulty in drawing, due to strong intermolecular hydrogen bonding caused by the molecular structure, when compared to the a tactic PVA.
It is known that the syndiotactic PVA fiber developed up to now has higher tensile strength and modulus at a lower draw ratio than the a tactic PVA fiber of the same molecular weight. Therefore, if the molecular orientation of the syndiotactic PVA having a compact structure is increased by increasing the draw ratio, the PVA fiber having higher tensile strength and modulus can be produced.
To prepare a conventional PVA short fiber poly(vinyl ester), the precursor of PVA is synthesized and this precursor is saponified to produce PVA. After the synthesized PVA polymer had been solution-spun or gel-spun, the molecular chain of the PVA filament is oriented in parallel by the additional drawings and heat treatment. The PVA short fiber(staple) is prepared by cutting the continuous PVA filament to a suitable length. The high strength Vinylon.RTM. Fiber (Kuraray Co., Japan), now commercially available, is prepared by this method.
The conventional PVA fiber may also be prepared according to Japanese Patent Laid Open Document 04-108109. A monomer having a side group giving steric hindrance is polymerized and a syndiotacticity-rich precursor is synthesized by the saponification of the polymer, followed by separating, washing, and drying. It is redissolved in the solvent, spun, drawn and dried. The PVA filament prepared by this process is cut by a special cutting-machine to obtain the short staple fiber.
According to U.S. Pat. No. 4,511,623, a short fiber can be prepared by polymerization with the help of the special action of pyridine/amide solvent, without spinning and drawing processes, in the course of preparing rigid rod poly(p-phenylene terephthalamide) known as aamid. However, this patent does not show preparing the short fiber accumulated with ultrafine microfibrils from the flexible chain polymer PVA in the course of saponification.
According to U.S. Pat. No. 5,238,995, a PVA polymer is prepared by the saponification of poly(vinyl pivalate) dissolved in tetrahydrofuran with the saponifying agent composed of a potassium hydroxide/methanol solution, and then the PVA is separated, dried and redissolved in the solvent. After spinning, drawing, heat-treatment, washing, drying, and cutting, the PVA short fiber is produced. PVA prepared by this method has a degree of saponification of 99 mol %, syndiotactic and content of over 60%, a high degree of orientation, and high crystallinity.
According to Japanese Patent Laid Open Document 04-117408 and 05-080215, and Yamamoto [T. Yamamoto et al., Polymer Journal, 23,185(1991)], poly(vinyl acetate) can be completely saponified by a general saponification method using sodium hydroxide/methanol solution. However, this syndiotacticity-rich high molecular weight polymer containing high molecular weight poly(vinyl pivalate) cannot be saponified effectively using sodium hydroxide. Hence, Yamamoto dissolved the syndiotacticity-rich poly(vinyl pivalate) polymer in tetrahydrofuran and synthesized the PVA with a hydroxyl group content of over 99 mol % by saponifying using potassium hydroxide/methanol solution. This saponification method has the advantage that the degree of saponification of over 99 mol % can be obtained without molecular chain scission. However, the syndiotactic PVA, which has a strong intermolecular hydrogen bonding as formula(c) indicates, is synthesized quickly, forming many hydroxyl groups in a very short time by a vigorous and speedy reaction of poly(vinyl pivalate) with the saponifying agent. ##STR2## The hydrogen bonding force of the syndiotactic PVA causes gel or precipitate formulation after the completion of saponification.
Adding iodine compound to the PVA film prepared by film casting to improve the drawability of PVA has been developed [Y. S. Choi, et al., Polymer Journal, 22 601 (1990)]. This added iodine compound plays a role in weakening the intermolecular hydrogen bonding by changing the crystal structure of PVA, thus improving the drawability of the polymer. As this compound is removed after the drawing, the original crystal structure of PVA is recovered and a highly oriented material is obtained. However, such a method should be applied in the course of or after spinning or film casting.
The present inventors have conducted long term investigations and studies to develop a new method which can produce a PVA microfibrillar short fiber having high tensile strength and modulus, omitting procedures such as dissolving PVA synthesized after saponification, spinning the above PVA in the form of solution or gel, drawing more than ten times, and heating at a high temperature.
The present inventors have realized that the high strength and high modulus PVA microfibrillar fiber which has different length, diameter, and elongation can be produced by adding a special alkali saponifying agent to the syndiotacticity-rich high molecular weight poly(vinyl pivalate)/tetrahydrofuran solution while being stirred with a special shearing device at a certain shear rate.
The object of this invention is to simplify the complex preparation process of PVA fiber and to provide a PVA microfibrillar fiber having high tensile strength and modulus, excellent alkali resistance, and good thermal stability.